1. Field of Invention
This invention is related to motion control of a tissue emulsifier needle and more particularly to the axial displacement pattern followed by the tip of an emulsifier needle used to emulsify biological tissues such as the phacoemulsification of the crystalline lens during eye surgery.
2. Description of Prior Art
Current phacoemulsification apparatus usually use electro-mechanic actuators driven at sonic or ultrasonic frequencies to energize a hollow phacoemulsification needle. These actuators operate under piezoelectric, magnetostrictive or voice-coil principles.
Axial sinusoidal oscillation of the phacoemulsification needle emulsifies the lens tissue of the eye during cataract surgery primarily by jackhammer action. Friction between the hollow phacoemulsification needle external walls and the surrounding elements generates undesired heat that can lead to thermal injury of the ocular tissues. Also high intensity ultrasonic activity produces undesired cavitation bubbles.
Several measures have been adopted to minimize the risk of thermal injury. For example, it is a necessary condition that cooling fluid circulates before ultrasonic power can be applied in order to guarantee sufficient heat dissipation. Also, an aspiration bypass port, consisting in a lateral shunt located near the distal opening has been added to refrigerate the surroundings of the phacoemulsification needle when the distal opening of the phacoemulsification needle becomes occluded by lens material.
Furthermore, various schemes of modulation of the driving signal that powers the ultrasonic electro-mechanic actuators have been used to reduce the thermal risk during the procedure. These schemes consider the intercalation of periods of actuator inactivity that provide thermal relaxation before the following period of ultrasonic oscillatory activity takes place, minimizing heat build up.
In this direction, Pulsed Mode is one of the basic power modulation schemes typically having a 50% duty cycle where a burst of ultrasonic activity is followed by an equal period of inactivity, usually providing up to 15 pulses per second.
Burst Mode is another scheme that provides an energized actuator for a fixed predetermined period, typically above 30 milliseconds duration, followed by an operator controlled period of actuator inactivity and repeated in cycles.
Recently, systems providing very short bursts in the order of 4 milliseconds have been introduced, with a user selectable separation between bursts. This method has been claimed to generate less heat and to provide more lens disruption power for similar ultrasound energy levels delivered into the ocular tissues. The explanation raised for the increased efficiency produced by this mode would consist in a relative increase of transient cavitation over steady cavitation.
Inventor's research on phacoemulsification needle interaction with lens tissue using high speed recording techniques has revealed that the most efficient portion of an ultrasonic burst to disrupt lens tissue occurs when the amplitude of oscillations increases at the initial portion of each single burst. This portion of a burst can be defined as the attack portion of the envelope of the motion burst.
Once the amplitude of oscillations attains the maximum preset steady amplitude, efficiency to disrupt tissue decreases to an intermediate level, as observed by the speed at which a fragment of cataract tissue advances into the phacoemulsification needle tip.
The same studies teach that during ultrasonic activity only a small portion of the tip axial excursion located at the distal end of each stroke has real contact with the lens tissue being emulsified. This occurs because the high repetition rate of each stroke does not allow a lens fragment to follow the full trajectory of the phacoemulsification needle because of fragment inertia.
The lens fragment remains at the distal zone of the excursion of the phacoemulsification needle tip during steady ultrasound. Only about 5 degrees of the sinusoidal trajectory located before the apex of the stroke of the phacoemulsification needle tip enters in contact with the lens fragment during steady ultrasonic activity.
This situation is different during the attack and decay portions of a single ultrasonic burst. During the attack portion the amplitude of ultrasonic oscillations progressively increases. Lens fragment inertia during the attack portion allows the phacoemulsification needle tip to produce a relatively higher lens disruptive effect when compared to the steady ultrasound lens disruptive effect.
In the opposite way, during the decay or ending portion of a single burst of ultrasonic activity, the lens disruptive effect is greatly reduced with respect to the ultrasound lens disruptive effect observed during steady ultrasound because contact between the needle tip and the lens fragment is diminished.
Object and Advantages. A main object of the present invention is to provide a method to increase the efficiency of phacoemulsification needles with respect to currently used methods of ultrasonic energy delivery for lens removal, allowing the effective use of less ultrasonic power and decreasing heat generation.
A further object of the present invention is to reduce the delivery of ultrasonic energy into the eye during the periods of ultrasonic activity known to provide minimal or no lens disruptive power.
An advantage of the present invention is to reduce the undesired thermal and cavitation effects when compared to currently used systems with similar degrees of lens removing capabilities. Further objects and advantages of the present invention will become apparent from consideration of the drawings and ensuing description.